1. Field of the Invention
This invention relates to a novel rubber composition having a high modulus of elasticity and excellent tear resistance, said composition consisting essentially of a rubber matrix having micro-dispersed therein fine particles of a high-melting isotactic poly-.alpha.-olefin, and a novel process for preparing said rubber composition.
2. Description of the Prior Art
In certain specific fields of industry, there have recently become desired vulcanized rubbers having a higher modulus of elasticity than the generally available vulcanized rubbers. In this case, it is desirable that the other physical properties than the modulus of elasticity are substantially tha same as those of the commercially available vulcanized rubbers and the processability in the production of vulcanized rubber articles is also excellent.
As a means for increasing the modulus of elasticity of vulcanized rubber, a method is known by which a material having a high modulus of elasticity (such as carbon black, resin, etc.) in a powdery or fibrous form is mixed with the rubber. However, the product of such a method is not satisfactory in dispersibility of the high modulus-of-elasticity material in the rubber, processability of the mixture and other physical properties of vulcanizate than modulus, for example, tear resistance and the like.
Attempts have been made for improving the modulus of elasticity and strength of rubber by micro-dispersing a resin in a rubber matrix, and it is disclosed that a rubber composition comprising a highly crystalline 1,2-syndiotactic polybutadiene shows a high modulus of elasticity and that a rubber composition having excellent tear resistance and also well balanced physical properties can be obtained by a certain specific method. For example, Japanese Patent Publication Nos. 17666/74 and 17667/74 disclose a two-step polymerization method which comprises cis-1,4-polymerization and subsequent 1,2- polymerization of 1,3-butadiene, and Japanese Patent Application Kokai (Laid-Open) No. 126050/78 discloses a method by which 1,2-syndiotactic polybutadiene powder is mixed with rubber and the mixture is treated at a temperature which is at least 5.degree. C. higher than the melting point (around 200.degree. C.) of 1,2-syndiotactic polybutadiene and extruded into the form of string or sheet. However, the former method has a difficulty in controlling the properties of the polymer mixture and is also subject to a restriction that the rubber must be cis-1,4-polybutadiene. On the other hand, the latter method, although open to use of various types of rubber, has problems in that it requires the specific processing conditions such as a high temperature required in the rubber processing operations and that the control of the processing temperature is difficult because 1,2-syndiotactic polybutadiene is liable to gel at elevated temperatures. Further, since the melting point of 1,2-syndiotactic polybutadiene is around 200.degree. C., there arises a problem in practical applications where a use temperature higher than the melting point of said polybutadiene is required.
The present inventors have conducted extensive research for obtaining a rubber composition having a high modulus of elasticity and excellent tear resistance without restricting the rubber to be used nor requiring any special processing conditions by incorporating thereinto a crystalline polymer superior in heat resistance to 1,2-syndiotactic polybutadiene, and as a result, it has been found that a novel rubber composition having a high modulus of elasticity and excellent tear resistance can be produced commercially advantageously by blending and micro-dispersing a high-melting isotactic poly-.alpha.-olefin in rubber by a specific method.